1. Field of the Invention
The invention relates to a driving force transmission apparatus that transmits driving force from an input shaft to an output shaft in, for example, an automobile.
2. Description of Related Art
There is an existing driving force transmission apparatus that is, for example, mounted on a four wheel drive and that couples a pair of rotating members by a clutch such that torque is transmittable (for example, see Japanese Patent Application Publication No. 2002-48157 (JP 2002-48157 A)).
The driving force transmission apparatus of this type includes an outer rotating member, an inner rotating member relatively rotatably supported coaxially with the outer rotating member, a main clutch that couples the outer rotating member to the inner rotating member such that torque is transmittable, an electromagnetic clutch mechanism arranged along a rotation axis in parallel with the main clutch, a cam mechanism that converts rotation force from the outer rotating member to pressing force toward the main clutch through actuation of the electromagnetic clutch.
The cam mechanism includes a pair of cam members, and is configured such that one of the cam members rotates relative to the other one of the cam members upon reception of rotation force from the electromagnetic clutch mechanism and then the other one of the cam members presses the main clutch as a result of the rotation.
The electromagnetic clutch mechanism includes an electromagnetic coil, an armature that receives electromagnetic force of the electromagnetic coil, and outer clutch plates and inner clutch plates that are pressed by the armature. The outer clutch plates are spline-engaged with the outer rotating member. The inner clutch plates are spline-engaged with the one of the cam members of the cam mechanism.
When the electromagnetic coil is energized, the armature presses the outer clutch plates and the inner clutch plates. Thus, the rotation force of the outer rotating member is transmitted to the one of the cam members, the rotation force is converted to axial pressing force, and the other one of the cam members presses the main clutch. Thus, the outer rotating member and the inner rotating member are coupled to each other such that torque is transmittable.
Incidentally, in order to ensure the strength, the cam members of the cam mechanism are formed of iron that is a magnetic material. Therefore, when the armature is close to the cam members at the time when the electromagnetic coil is energized, the cam members may form a magnetic path, and the armature may be attracted toward the cam members. In this case, the outer clutch plates and inner clutch plates of the electromagnetic clutch mechanism are not pressed, so the cam mechanism is not actuated, and the outer rotating member and the inner rotating member are not coupled to each other such that torque is transmittable.
In order to solve the above inconvenience, the driving force transmission apparatus described in JP 2002-48157 A includes a clearance regulating member formed of a non-magnetic material for ensuring a clearance between the armature and the cam members. The clearance regulating member specifically, for example, includes balls formed of a non-magnetic material and respectively fitted to fitting holes formed in the cam members or a leaf spring arranged between the cam members and the armature. Movement of the armature toward the cam members is regulated by the clearance regulating member, and the armature is reliably attracted toward the electromagnetic coil at the time when the electromagnetic coil is energized.
However, the driving force transmission apparatus described in JP 2002-48157 A additionally includes the clearance regulating member, so the number of components increases, and the number of assembling man-hours also increases. In this respect, there is still room for improvement.